1. Field of the Invention
This invention relates to a thermoplastic resin composition having excellent impact resistance, weather resistance and moldability, as well as to a process for preparing the same.
2. Description of the Prior Art
Among impact-resistant resins is ABS resin which forms a two-phase resin composed of resin and rubber. However, it is well known that ABS resin is subject to deterioration by ultraviolet light or the like and hence exhibits poor weather resistance, because the butadiene-based polymer consituting its rubber component for imparting impact resistance to the resin has a large number of chemically unstable double bonds in its backbone.
In order to improve the weather resistance of ABS resin, there has been proposed a process for the preparation of a resin composition by using a saturated rubbery polymer having few double bonds in its backbone. One typical example is a resin composition containing an acrylic ester-based rubber. Although this saturated rubber is stable to ultraviolet light, it is difficult to establish therein a crosslinked rubber or grafted structure which is an essential requirement for two-phase resins composed of resin and rubber, because the rubber has no active site for cross-linking or grafting. Thus, such resin compositions have the disadvantages of being softer, having a lower elastic modulus, and exhibiting slower elastic recovery as compared with those containing a diene rubber. For these reasons, when a resin composition analogous to ABS resin but containing such a saturated rubber is used as an injection molding material, the rubber particles exhibit a marked orientation, producing an iridescent luster on the entire surfaces of molded articles or in a definite direction of flow of the material. Moreover, when such a resin composition is colored with a pigment or the like, the above-described tendency becomes more pronounced and thereby causes a decrease in commercial value. In order to overcome these disadvantages, there have been proposed a process involving copolymerization in the presence of a selected cross-linking agent, a process involving peroxide cross-linking, processes using a composite-structured crosslinked acrylic containing a diene rubber within the particle, as disclosed in Japanese Patent Publication No. 47863/'72, Japanese Patent Laid-Open Nos. 86918/'81, 133311/'81, 167308/'82 and 120663/'83, and U.S. Pat. No. 4,393,172, and the like.
However, in thermoplastic resins proposed in the prior arts, the balance between the gloss and impact resistance of molded articles obtained by molding at high temperatures is not quite satisfactory. Specifically, such thermoplastic resins have the disadvantage that, when they are molded in a wide temperature range extending from low to high temperatures, the resulting molded articles cannot always retain a high gloss. Thus, a thermoplastic resin having a combination of excellent impact resistance, weather resistance and moldability has not been available in the prior arts.
In view of these circumstances, the present inventors conducted an intensive research for the purpose of improving the balance between the impact resistance and gloss of molded articles formed under molding conditions ranging from low to high temperatures and have found that the above-described problems can be solved, i.e., a thermoplastic resin composition having a combination of excellent impact resistance, weather resistance and moldability can be obtained, by forming a graft copolymer resin through polymerization of at least one monomer selected from the group consisting of aromatic vinyl compounds and ethylenically unsaturated compounds, in the presence of a latex containing a composite-structured acrylic rubber whose particles are composed of a core consisting of an enlarged diene rubber prepared by treating a diene rubber latex of smaller particle size with an acid group-containing copolymer latex, and an outer layer consisting of a crosslinked acrylate polymer derived from an acrylic ester as the main component and formed by the combined use of a graft-linking agent and a cross-linking agent. The present invention has been completed on the basis of this finding.